The present invention relates to an engine control system for cleaning exhaust gas exhausted from lean-burn combustion engines, particularly, for suppressing release of NOx, and to an engine control device for achieving always stable clean up of the exhaust gas by controlling reactivation of catalyst based on an estimation of deterioration of the catalyst performance.
Regarding an improvement on clean-up rates of lean NOx catalyst, JP-A-5-133260 (1993), for instance, discloses a method for improving transient clean-up rates by changing a target air-to-fuel ratio from a rich condition to a lean condition and its reverse alternately. However, the above prior art does not teach nor suggest any reactivation (recovery) control of the clean-up rates to SOx poisoning based on time-elapsing effects of NOx catalyst by heating utilizing so-called after-burn efficiencies and the like.
Regarding conventional in-cylinder-injection engines, a prior art is described, for instance, in JP-A-4-241753 (1992). The technology is to make a fuel distribution homogeneous by controlling fuel injection timing based on a temperature of cooling water of the engine, in order to achieve stable stratified combustion in a lean condition of the in-cylinder-injection engine.
However, any special countermeasures for the exhaust gas of the stratified combustion, wherein the combustion is performed in a lean condition far from the stoichiometric air-to-fuel ratio, has not been considered. Regarding release of NOx, which is a particular problem of the lean combustion, no processing nor controlling has been considered.
In accordance with the lean-burn engine, particularly lean combustion (stratified combustion) in the in-cylinder-injection engine, the combustion is performed by forming a combustible mixture locally in the vicinity of plugs in a remarkably lean condition as a whole cylinder.
Accordingly, its exhaust gas can not be cleaned on all the components to be removed by conventional three way catalyst, particularly, a lean NOx catalyst becomes necessary for nitrogen oxides (NOx).
Regarding the NOx catalyst, an important point is how to maintain its performance. In particular, a countermeasure for gasoline containing sulfur components (S) more than conventional gasoline becomes necessary in accordance with deregulation on gasoline and the like.
Even on the conventional gasoline, a decrease of the clean-up rate by sulfur can be generated based on transient effects, and it is regarded as a SOx poisoning.
The SOx poisoning is caused by S components in gasoline, and depending on how much the catalyst itself is exposed to the exhaust gas atmosphere.
FIG. 3 indicates a change of characteristics of the lean NOx catalyst in accordance with elapsing time, which is indicated on the abscissa. In accordance with the characteristics, it is revealed that the change is proportional to the time exposed to the exhaust gas atmosphere, in other words, proportional to a total mount of gasoline containing S supplied to the engine.
Furthermore, in accordance with the mechanism, the S component in gasoline is changed to SOx (sulfur compounds) by combustion, released as a part of the exhaust gas, and formed a compound with an active component for NOx clean-up in the lean NOx catalyst at surface of the catalyst, as shown in FIG. 4. Due to the above phenomenon, the function of the active component for NOx clean-up in the catalyst is decreased, and the catalyst performance is deteriorated.
On the other hand, it has been known that the clean-up performance of the catalyst can be recovered by making its atmosphere at a designated temperature as indicated by dotted lines in FIG. 3. In the above case, the higher the temperature in the atmosphere of reactivation control shown by an arrow is, the higher the degree of the recovery is.
The mechanism of recovery is to make the active component of the catalyst functional as it conventionally is by heating the catalyst (for instance, at least 500xcx9c600xc2x0 C.) for separating SOx from the active components of the catalyst.
The problem to be solved by the present invention is to provide an engine control device provided with lean NOx catalyst, which is usable of the lean NOx catalyst having the above characteristics in a preferable condition at all the time, and to provide an engine control device provided with the lean NOx catalyst, which can practically perform the reactivation control indicated by the arrow in FIG. 3 when the clean-up rate is decreased by sulfur and the like based on change of the lean NOx catalyst with elapsing time.
The engine control device provided with the lean NOx catalyst relating to the present invention solves the above problems by the following measures.
First, the engine control device of the present invention comprises a condition estimating means and a reactivation controlling means for catalyst itself.
The condition estimating means for catalyst has practically a following composition. A deteriorated condition of the catalyst depends on the elapsing time exposed to the exhaust gas, and the degree of deterioration depends on the amount of fuel used for the combustion. This is clear from the previously described mechanism of deterioration by S components contained in the fuel. Accordingly, the degree of deterioration is estimated from the total supplied amount of the fuel and the elapsing time of the catalyst exposed to the exhaust gas containing the S components. Furthermore, the degree of deterioration of the catalyst can be determined by any of the temperature of the catalyst itself, O2 sensors arranged at upstream and downstream of the catalyst, and various exhaust gas sensors such as air-to-fuel ratio sensors and the like.
Next, the reactivation control at a high temperature can be achieved by elevating the exhaust gas temperature forcibly in view of the fact that the engine is operated routinely in a lean condition and the exhaust gas temperature is lower than that in a conventional condition of stoichiometric air-to-fuel ratio.
Practically, the exhaust gas temperature is elevated by an ignition timing retardation control. Regarding the in-cylinder-injection engine, the exhaust gas temperature can be elevated with an after-burn effect by performing fuel injection during an exhaust stroke. In accordance with the control, the exhaust gas temperature can be elevated forcibly to higher than a routine operation condition, and the reactivation control indicated in FIG. 3 becomes possible. It is clear that the ignition timing retardation control is applicable to conventional lean combustion of a MPI system, and that the reactivation control is effective for achieving a heating effect for the catalyst atmosphere, because fuel combustion becomes possible without using a control such as a special ignition control and the like by injecting fuel into an atmosphere at a relatively high temperature during exhaust valves are in an opening condition during an exhaust stroke injection of the in-cylinder-injection engine.
Controlling time for the reactivation control is more effective with a shortened time at a higher temperature as the dotted line in FIG. 3 indicates. Accordingly, the controlling time of the reactivation control is regulated based on an estimated exhaust gas temperature in that condition. The degree of the heating effect in the reactivation control is the largest at the first cycle, and is in a tendency to decrease after second cycle with the same heating time. Therefore, in order to realize the reactivation control more effective, it is effective to change the length of the controlling time in consideration of the controlling cycles such as extending the controlling time after second cycle longer than the controlling time of the first cycle.
When the reactivation control become necessary in a stratified combustion condition of a lean condition, the reactivation control is performed by replacing a fundamental combustion condition of the engine with an air-to-fuel ratio condition. In accordance with the replacement, the reactivation control is performed in a condition that the basic exhaust gas temperature is elevated by injecting at an exhaust stroke and retarding the ignition timing in order to improve its effects, and concurrently, the basic temperature during the reactivation control can be made clear by selecting the stoichiometric air-to-fuel combustion condition as the standard condition of the exhaust gas temperature. Instead of the estimated exhaust gas temperature, the exhaust gas temperature determined by temperature sensors arranged in the lean NOx catalyst or the exhaust gas system including the lean NOx catalyst can be used.
FIG. 2 indicates a fundamental portion of the present invention. A Sox poisoning amount of the catalyst is estimated based on Qa, i.e. an amount of air flow intake into the engine, or Ti, i.e. a pulse width for controlling the amount of injection supplied into the engine. Simultaneously, activation of the exhaust gas injection is judged based on the exhaust gas temperature, and the reactivation control is performed with monitoring the exhaust gas temperature by fuel and ignition control.
During the reactivation control, the exhaust gas system damage (thermal deterioration by a high temperature) including the lean NOx catalyst by a careless control must be avoided, because the exhaust gas temperature is increased forcibly.
The heating condition for recovering the functions of the active component of the catalyst requires at least 500xc2x0 C. as described previously. However, the active component of the catalyst has a possibility to cause a thermal deterioration if the heating temperature exceeds approximately 900xc2x0 C. Therefore, the reactivation control must be performed so that the lean NOx catalyst temperature, or the exhaust gas temperature at the upstream entrance portion of the lean NOx catalyst must be kept lower than 900xc2x0 C.
If an overshoot, wherein the transient temperature exceeds 900xc2x0 C. in a course of stabilizing the reactivation control, and the like are generated, an unit continuous control time of the reactivating over 900xc2x0 C. is desirably shorter than tens to 30 seconds. If a case when the unit time exceeds tens to 30 seconds, the reactivation control must be stopped, or interrupted temporarily, and the reactivation control is desirably resumed at a temperature below 900xc2x0 C.